The present invention relates to a protect layer transfer sheet in which a protect layer is disposed on a substrate to be separable therefrom, and particularly relates to a protect layer transfer sheet which can surely transfer a protect layer onto a printed product, thereby providing durability to an image previously formed on a surface of the printed product through a thermal transfer recording or another transfer recording process.
The present invention also relates to a protect layer to protect a quality image on a printed product obtained through any of transfer recording processes such as a sublimation thermal transfer process, an ink jet printing process or the like.
There have been known thermal transfer methods such as a sublimation (type) thermal transfer method and a heat fusion (type) thermal transfer method. The sublimation thermal transfer method is carried out by: using a sublimation thermal transfer sheet in which a dye layer containing a sublimation dye and a binder is formed on a substrate film; laying the sublimation thermal transfer sheet on a transfer-receiving material; heating the sublimation thermal transfer sheet by a heating means such as a thermal head or a laser beam in accordance with information or signals for transferring of an image in order to sublimate the dye from the dye layer and transfer it to the transfer-receiving material, thereby recording or outputting the image. On the other hand, the heat fusion thermal transfer method is carried out by: using a heat fusion thermal transfer sheet in which a heat fusible ink layer containing a coloring material such as pigment and a vehicle such as wax is formed on a substrate film; laying the heat fusion thermal transfer sheet on a transfer-receiving material; heating the heat fusion thermal transfer sheet by a heating means similar to that in the sublimation thermal transfer method in order to soften or fuse a ink in the heat fusible ink layer and transfer it to the transfer-receiving material, thereby recording or outputting the image.
The sublimation thermal transfer method of the former is a particularly excellent method to form a precise and beautiful image having a gradational tone such as a photograph of a portrait. The heat fusion thermal transfer method of the latter is particularly suitable for a case where a simple image such as a letter, a numeral or the like is easily and clearly formed. Various kinds of images is easily formed by these thermal transfer methods, and hence the thermal transfer methods are getting preferably utilized for printing a printed product in which a relatively small amount of copies are merely required, for example, a card such as an identification card, various kinds of certifications, or a portrait of the deceased, which often has a large size with completely reproduced colors.
However, since the image formed through the sublimation thermal transfer has no vehicle, it is inferior in durability such as light resistance, weather resistance, wear and abrasion resistance, chemical resistance, solvent resistance or the like to an image formed with an usual ink. In addition, though the image formed through the heat fusion thermal transfer method is supported by the vehicle, it is also inferior in durability to the image formed with an usual ink, and particularly inferior in the wear and abrasion resistance.
There has been known a method for improving the durability of the printed product formed through the thermal transfer process, which is carried out by: laying a protect layer transfer sheet provided with a resinous layer having a thermally transferability, i.e., a thermally transferable protect layer, over an image previously formed through the sublimation or the heat fusion thermal transfer method; and transferring the resinous layer by a heating means such as a thermal head or a heating roll, thereby forming a protect layer on the image. When the protect layer is formed over the image of the printed product, the image can be improved concerning the wear and abrasion resistance, the chemical resistance, the solvent resistance or the like. The light resistance of the image can also improved by adding an ultraviolet absorbent into the protect layer. Furthermore, the protect layer may effect an extra function such as prevention of falsification or counterfeitness, improvement in whiteness of the printed product or the like if a fluorescent whitening agent or the like is added into the protect layer.
In order to surely and constantly transfer the protect layer on the image of the printed product at all times, the protect layer formed on a substrate of the protect layer transfer sheet is required to be quickly and surely transferred on a surface of the image and bonded thereto when the protect layer transfer sheet is heated from its back surface by the heating means such as the thermal head, the heating roll or the laser beam. Thus there has been made an attempt to improve transferability of the protect layer by disposing a transferable protect layer on a substrate of the protect layer transfer sheet by the medium of a release layer.
When the release layer is utilized for improving the transferability of the protect layer, a primary requirement is that a boundary portion between the substrate of the protect layer transfer sheet and the release layer always has a sufficiently large adhesive strength in comparison with that between the transferable protect layer of the protect layer transfer sheet and the release layer. If a relationship between the adhesive strengths of the two boundary portions becomes reverse to that as described above, the release layer which ought not to be transferred will be transferred on the printed product together with the protect layer. That is, a phenomenon called as "robbing of the release layer" is caused.
In addition, it is desirable that the adhesive strength between the release layer and the transferable protect layer is sufficiently large when a heating energy is not applied, and it becomes sufficiently small when the heating energy is applied. If the adhesive strength between the release layer and the transferable protect layer is not enough before applying of the heating energy, an unintentional peeling off of the protect layer or a defective edge of the transferred protect layer (i.e., deterioration of a sharpness of the edge of the transferred protect layer) may be caused. If the adhesive strength described above is excessive at the applying of the heating energy, a noise of the peeling or a sticking may be caused during the transferring process of the protect layer, and furthermore a defect of transferring or conveying may be caused in worse case.
Further, it is desirable that an adhesive strength between the release layer and the transferable protect layer does not depend on temperature and humidity of environment. In case the adhesive strength between the release layer and the transferable protect layer depends on the temperature and the humidity, it causes the defects of the protect layer transferring when the layers are preserved under the condition of high temperature or high humidity for a long period of time, or the protect layer is transferred under the condition of high temperature or high humidity.
The conventional protect layer transfer sheet however has the same tendency in the adhesive strength between the release layer and the transferable protect layer at all times whether the heating energy is applied or not. More specifically, if the adhesive strength between the release layer and the transferable protect layer is made large before the applying of the heating energy, that after the applying of the heating energy will also increase. On the other hand, if the adhesive strength described above is made small after the applying of the heating energy, that before the applying of the heating energy will also decrease. Therefore it has not been possible to obtain a release layer or a transferable protect layer which has adhesive characteristics desirably changeable as stated above.
Further, when the conventional protect layer transfer sheet is used, the condition of preserving the protect layer transfer sheet and transferring has to be greatly limited for transferring the protect layer excellently, because the adhesive strength between the release layer and the transferable protect layer is varied in accordance with the temperature and the humidity during preservation and transferring.
In another aspect, the sublimation thermal transfer method among the thermal transfer recording method is particularly coming capable of forming a beautiful image having gradational tones with a high reproducibility, which is comparable to a photograph. An ink jet system of the transfer recording method is also coming to realize a high quality of the image comparable to the photograph. Therefore such transfer recording methods as can obtain the highly quality image are developing as a technique substitutive for a silver photograph. However, the image formed through the sublimation thermal transfer method or the ink jet method is inferior in the durability to the image formed through the silver photograph method.
As one method to improve the durability of the image formed through the transfer recording method, there has been known a method described above in which the image is covered with the protect layer through thermal-transferring the protect layer from the protect layer transfer sheet. In the conventional protect layer transfer sheet, the thermally transferable protect layer is laminated on the substrate by the medium of the untransferable release layer in order to improve the transferability of the protect layer, and furthermore an adhesive layer is formed as an uppermost layer of the protect layer in order to improve an adhesive property of the transferable protect layer to a surface of the printed product. In addition, since the conventional protect layer transfer sheet has a principal objective to protect a surface of a card such as an identification card or a credit card, it is required to have a high durability as endures a relatively severe condition, such as a high resistance to rubbing, a resistance to plasticizer desired at the time when the card is put in a holder of polyvinyl chloride and left at a high temperature. In order to ensure the high durability, the conventional thermally transferable protect layer is often formed by laminating plural layers each of which mutually has a different function. Therefore the transferable protect layer of the conventional protect layer transfer sheet has a multi-ply structure, because the protect layer has at least the adhesive layer in addition to a main layer or a main ply, and furthermore another layers or plies may be incorporated for providing plural function to the protect layer.
However, the multi-ply structure of the protect layer is apt to deteriorate a transparency of the protect layer and decrease the highest density of the image, thereby being not desirable for protection of a highly quality image. Furthermore the multi-ply structure of the transferable protect layer is inferior in a heat dissipation from the protect layer transfer sheet during the thermal transfer process, thereby deteriorating the sharpness of the edge of the transferred protect layer.
In addition, when the transfer recording method is carried out in place of the silver photograph technique, the high durability as desired in the cards is generally not required. For example, if a printed product formed through the transfer recording method is stuck in an album and preserved, it is enough for the printed product to have a fingerprint resistance (i.e., a resistance to a sebum) or a rubbing resistance enduring a light friction such as rubbing of a eraser at an ordinary temperature. Therefore the conventional transferable protect layer having the multi-ply structure has an excessive performance when it is utilized as the substitutive technique for the silver photograph, resulting in a deterioration of productivity and a rise of cost.